Conventional motes are small electronic devices that sense at least an element of their surroundings and communicate information related to the element sensed. A mote may communicate just the information it has collected, or such information may be combined with that from other motes and communicated, for example, to a central device.
Conventional motes communicate wirelessly with one another and/or a central device. If the device or other mote with which the mote wishes to communicate is out of range, the mote may communicate via a multi-hop communication through other motes.
Because some or all motes may rely on a small source of stored power such as batteries, it can be desirable for a mote to conserve power. One way motes draw power is during receipt of transmissions. To keep a radio on all the time to receive any transmission can represent a substantial power drain on the mote's power supply.
One conventional way that power could be conserved is to divide the time into slots, and require the motes to leave their radios off, turning them on to listen only during some of the slots. The radios are off during other slots, conserving power. If the start of a transmission is received during one of the slots in which the mote is to be listening, the mote can keep the radio on until the entire transmission is received. A beacon is periodically transmitted to synchronize the clocks of the different motes. However, this arrangement is difficult to synchronize in a multi-hop environment.
To conserve power in a multi-hop system, some systems have one mote that wishes to communicate with another to transmit a repeating preamble signifying that a message is forthcoming. The preamble is repeatedly transmitted throughout a period in which each device turns on their radio and listens for a preamble. If the device does not detect a preamble, it turns off its radio for one preamble period, thereby conserving power. If the device detects a preamble, the device keeps its radio on until the actual communication is received. The technique thus requires the transmitter to make additional transmissions over that used by a timeslot arrangement, but reduces the power used by the receivers. Where transmissions are infrequent, the system as a whole saves power, although the power required for each transmission rises.
Recently an IEEE communication standard known as 802.15.4 has been adopted by designers of communication products, and many conventional mote manufacturers would like to use the standard for inter-mote communication. An improvement on the preamble technique that can be used with 802.15.4 communications and other types of communications is described in the related application. That application describes an arrangement in which a series of “chirp” packets is sent before one or more data packets are sent, like the preamble technique described above. However, unlike the preamble technique, each chirp packet identifies a time at which the data packet or packets will be sent. The chirp packets are sent throughout the course of a sleep period, during which other devices “sleep” in a low or no power mode, waking only for so long as it takes to hear one chirp packet during this period, and then sleeping until the data packet is received. This allows those receiving devices to conserve power during times other than when they are checking for chirp packets or receiving data packets.
Although this arrangement conserves the power of the receiving devices beyond that of the preamble technique, the sender continues to transmit a repeating chirp packet using approximately as much power as a the transmission of a repeating preamble, or even more. The sender may thus run out of power. Alternatively, the receivers may run out of power because receiving each packet may require the listening for, receipt of, a chirp packet.
What is needed is a system and method that can use the facilities of an 802.15.4-compatible device, but can allow a sender or receiver to be less likely to run out of power yet can be more efficient than an arrangement in which the receipt of a preamble causes a receiving device to keep its radio on until the actual communication is received, and does not involve a time-division multiplexing arrangement.